1. Field of the Invention
This invention relates to an electroplating solution for forming Pb--Sn alloy bump electrodes on a surface of a semiconductor wafer, which each have a uniform height and a smooth surface, and contain 30 ppb or less radioactive isotopes mainly including U, Th, and/or decayed nuclides thereof (hereinafter referred to simply as "radioactive isotopes") such that radioactive .alpha.-particles radiated from the isotopes, which can cause soft error, can be reduced to 0.5 cph/cm.sup.2 or less in terms of a count number thereof.
2. Prior Art
Conventionally, it is generally employed to form, by electroplating, bump electrodes of a Pb--Sn alloy (solder), each having a size of 20 to 250 .mu.m in diameter and 20 to 100 .mu.m in height on a surface of a semiconductor wafer, such as a Si wafer and a Ga--As wafer, in a number of 10,000 to 50,000 pieces per wafer, as described in "Microelectronics Packaging Handbook", pages 368 to 371, published by VAN NOSTRAND REINHOLD in 1989.
Pb--Sn alloy bump electrodes formed by the use of a conventional electroplating solution, however, are not satisfactory in that with recent increases in size of semiconductor wafers and degree of integration of circuits formed on the semiconductor wafers (packaging density), bump electrodes formed on a surface of a semiconductor wafer show variations in height therebetween, and degraded surface smoothness, exhibiting, in the worst case, a hedgehog-like surface or a rugged surface.